Fish virus-induced interferon exerts antiviral function through Stat1 pathway

Virus-induced interferons (IFNs) have been identified in various fish species and display antiviral activities similar to mammalian type I IFNs. However, apart from the mammalian IFN system, the IFN signaling pathway remains largely unknown. Using transient transfection and recombinant protein, we are reporting in this study that a crucian carp (Carassius auratus L.) IFN exhibits strong antiviral activity against grass carp hemorrhagic virus (GCHV) infection and also mediates Poly I:C-induced antiviral response, which correlates with its ability to induce a set of IFN-stimulated genes (ISGs). Strikingly, overexpression of wild-type Stat1 increases the effects of IFN on both the expression of ISGs and the inhibition of virus infection, whereas a dominant negative mutant of Stat1 (Stat1-ΔC), which lacks of the C-terminal transcriptional activation domain (TAD), inhibits the antiviral activity of IFN and reduces the expression of ISGs, demonstrating that fish IFN induces the expression of ISGs and host antiviral response through Stat1 pathway reminiscent that of mammalian IFNs. Significantly, unlike mammalian type I IFNs, recombinant fish IFN is able to upregulate IFN itself, which is enhanced by overexpression of Stat1 but impaired by knockdown of Stat1, indicating a positive feedback loop in regulation of IFN itself. These results provide strong evidence for the existence of an evolutionary conserved Stat1 pathway between fish and mammals, which is indispensable for fish virus-induced IFN antiviral response.